1. Field of the Invention
The present invention relates to a Raman amplifier that is used in an optical communication system and that amplifies signal light in an optical fiber.
2. Related Background Arts
When light is incident on an optical fiber, the light is scattered by the vibration of glass structure of the optical fiber, thereby yielding scattering components on the longer wavelength side and the shorter wavelength side relative to the wavelength of the incident light. The scattering component on the longer wavelength side is called a Stokes line, and the scattering component on the shorter wavelength side is called an anti-Stokes line. The power of the Stokes line is stronger than that of the anti-Stokes line. If signal light having the same wavelength as the Stokes line is incident on an optical fiber at the same time as Stokes line occurs, stimulated Raman scattering is generated. The signal light is thereby Raman-amplified while propagating through the optical fiber. That is, the light performs as pump light for Raman amplification. This phenomenon is used for obtaining a Raman amplifier of discrete type or distributed-constant type. A rare-earth-doped optical fiber amplifier is suitable for amplifying light having a wavelength that corresponds to the energy level, whereas a Raman amplifier is characterized in that it can Amplify signal light having any wavelength by selecting a suitable wavelength of pump light for Raman amplification.
In the case of an optical fiber made of silica glass as a main component, the transmission loss becomes the smallest around the 1.55 μm wavelength. Also, the strongest stimulated emission occurs at the wavelength that is 14 THz (about 100 nm) away on the longer wavelength side relative to the pump light.
On the other hand, to avoid the waveform degradation of signal light pulses due to four-wave mixing which is one kind of nonlinear optical phenomenon, the wavelength of signal light and the zero dispersion wavelength of an optical fiber must not overlap each other. Moreover, to allow signal light to be received at an acceptable S/N ratio at the receiving end, and also to avoid the waveform degradation of signal light due to four-wave mixing or cross-phase modulation which is one kind of nonlinear optical phenomenon, the wavelength of signal light and the wavelength of pump light for Raman amplification must not overlap each other.
Under such restrictions an optical communication system proposed at present that uses a Raman amplifier employs the 1.55 μm band as the spectrum band for signal light, using an optical fiber having a finite chromatic dispersion of a few ps·nm−1·km−1 at the 1.55 μm wavelength (i.e. non zero dispersion shifted optical fiber). It also employs pump light for Raman amplification near the 1.45 μm wavelength. The zero dispersion wavelength of the non zero dispersion shifted optical fiber is near the 1.50 μm wavelength.